Power supply

ABSTRACT

There is provided a power supply including a direct current (DC) to DC converter supplying main power to a load, and a sub converter connected to the DC to DC converter and reducing output loss, wherein the sub converter is operable based on a hold-up time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No.10-2012-0151302 filed on Dec. 21, 2012, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply operable during ahold-up time and thus capable of reducing output loss.

2. Description of the Related Art

In general, a phase shifted full bridge (PSFB) converter is widely usedas a direct current (DC) to DC converter in a power supply. This is dueto the fact that such a PSFB converter places less stress on asemiconductor device and allows zero voltage switching, and is thussuitable for high capacity applications.

The DC to DC converter needs to meet hold-up time requirements. That is,since a load is to be powered for a certain period of time, even if aninput alternating current power supply is interrupted due to failure,the DC to DC converter supplies power from the direct current linkvoltage charged in a capacitor in the input terminal of the powersupply. However, the direct current link voltage is reduced over time,and thus a duty ratio is required to be increased in order to supply aconstant amount of power to the load by taking the reduction in thedirect current link voltage into consideration.

Accordingly, the DC to DC converter needs to be designed to accept awider range of input voltage levels, and consequently it has a smallduty ratio in a normal state (i.e., a nominal state).

Therefore, the PSFB has the problem in that efficiency in supplyingpower is lowered.

Patent Document 1, referenced below, relates to a PSFB, but does notteach operating during a hold-up time to reduce loss.

RELATED ART DOCUMENT

(Patent Document 1) Korean Patent Laid-Open Publication No. 2011-0064605

SUMMARY OF THE INVENTION

An aspect of the present invention provides a power supply capable ofincreasing efficiency.

According to an aspect of the present invention, there is provided apower supply, including: a direct current (DC) to DC converter supplyingmain power to a load; and a sub converter connected to the DC to DCconverter and reducing output loss, wherein the sub converter isoperable based on a hold-up time.

The DC to DC converter may include: a primary circuit including aprimary winding of a transformer; and a secondary circuit including asecond winding magnetically coupled to the primary winding of thetransformer.

The primary circuit of the DC to DC converter may include a switchingmodule in which two terminals, one terminal of a first switching elementand one terminal of a second switching element connected in series, areconnected to both terminals of a voltage source, and two terminals, oneterminal of a third switching element and one terminal of a fourthswitching element connected in series, are connected to the bothterminals of the voltage source in parallel, wherein the primary windingof the transformer is connected between a first node and a second node,wherein the first switching element and the second switching element areconnected at the first node, and the third switching element and thefourth switching element are connected at the second node.

The secondary circuit of the DC to DC converter may include a switchingelement connected to the secondary winding of the transformer in series,and controlling current flowing in the secondary winding of thetransformer.

The sub converter may include: an inductor element connected to thesecondary circuit; a first sub switching element providing a path forenergy to be stored in the inductor element; and a second sub switchingelement providing a path for delivering energy stored in the inductorelement.

The first sub switching element may be turned on when the thirdswitching element is turned on, and may be turned off while the thirdswitching element remains turned on.

The second sub switching element may be turned on after a predeterminedperiod of time from when the first sub switching element is turned off,and may be turned off when the third switching element is turned off.

The first sub switching element may be turned on when the fourthswitching element is turned on, and may be turned off while the fourthswitching element remains turned on.

The second sub switching element may be turned on after a predeterminedperiod of time from when the first sub switching element is turned off,and may be turned off when the fourth switching element is turned off.

According to another aspect of the present invention, there is provideda power supply, including: a DC to DC converter supplying main powerthrough a primary circuit connected to terminals of a voltage source anda secondary circuit magnetically coupled to the primary circuit; and asub converter storing energy supplied from the secondary circuit andproviding a path for delivering the stored energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a circuit diagram of a power converter according to anembodiment of the present invention;

FIG. 2 is a graph illustrating operational wave forms of main componentsof a circuit in a nominal mode;

FIG. 3 is a graph illustrating operational wave forms of main componentsof a circuit in a hold-up mode;

FIGS. 4A to 4C are circuit diagrams illustrating the operation of acircuit in a nominal mode; and

FIGS. 5A to 5D are circuit diagrams illustrating the operation of acircuit in a hold-up mode.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings. The embodiments of thepresent invention may be modified in many different forms and the scopeof the invention should not be limited to the embodiments set forthherein. Rather, these embodiments are set forth to provide thorough andcomplete understanding of the present invention, and will fully conveythe concept of the invention to those skilled in the art. In theaccompanying drawings, shapes and dimensions of elements may beexaggerated for clarity.

FIG. 1 is a circuit diagram of a power converter according to oneembodiment of the present invention.

FIGS. 2 and 3 are diagrams showing waveforms of main elements of a powerconverter according to an embodiment of the present invention.

A power converter according to an embodiment of the present inventionmay include a main converter 100 supplying main power to a load, and asub converter 110 reducing power loss of the main power. For example,the main converter 100 may be a phase shifted full bridge direct current(DC) to DC converter supplying the main power to the load.

The sub converter 110 may be integrated into the main converter 100. Thesub converter 110 may be operable during a hold-up period in whichalternating current (AC) loss occurs, so as to reduce output loss.

Hereinafter, the power converter according to the embodiment of thepresent invention will be described in detail with reference to FIG. 1.

Referring to FIG. 1, as an example of the main converter 100 accordingto the embodiment of the present invention, a phase shifted full bridgeDC to DC converter (PSFBC) incorporating a sub converter 110 at itsoutput is shown. The phase shifted full-bridge DC to DC converter hashigh efficiency due low current/voltage stress and zero voltageswitching (ZVS), and thus is very advantageous for power applications.

More specifically, the main converter 100 includes: a bridge circuit Q₁to Q₄ in which two terminals, one terminal of a first switching elementQ₁ and one terminal of a second switching element Q₂ connected inseries, are connected to a voltage source Vs in parallel, and twoterminals, one terminal of a third switching element Q₃ and one terminalof a fourth switching element Q₄ connected in series, are connected tothe voltage source Vs in parallel; a transformer 101 and 102 including aprimary winding 101 connected between a first node N1, in which thefirst switching element Q₁ and the second switching element Q₂ areconnected, and a second node N2, in which the third switching element Q₃and the fourth switching element Q₄ are connected, and at least onesecondary winding 102 magnetically coupled to the primary winding 101;and an inductor element Lo and a capacitor element Co connected to thesecondary winding 102 of the transformer 101 and 102.

In addition, the main converter 100 may include switching modules Q₅ andQ₆ for allowing or interrupting current flows i_(Q5) and i_(Q6) in thesecondary winding 102 of the transformer 101 and 102. The sub converter110 may be disposed between both terminals of the transformer 101 and102 and both terminals of the capacitor element Co. The sub converter110 may include the inductor element Lo connected to the secondarywinding, a first sub switching element Q_(B1) providing the inductorelement Lo with a path for energy to be stored, and a second subswitching element Q_(B2) providing the inductor element Lo with a pathfor delivering the stored energy. According to the embodiment of thepresent invention, the sub switching elements allow or interrupt thecurrent flows i_(QB1) and i_(QB2) from the secondary winding 102 of thetransformer 101 and 102.

The turns ratio of the transformer 101 and 102 may be Np:Ns=n:1, and theprimary winding 101 may be represented by leakage inductance andmagnetizing inductance components L_(lkg) and Lm as shown in FIG. 1.Meanwhile, each of the first switching element Q₁ to the fourthswitching element Q₄ may include the respective one of diodes D1 to D4and the respective one of parasitic capacitors C1 to C4.

The main converter 100 thus configured supplies main power to the loadRo in a nominal mode and a hold-up mode.

In the following, the configuration including at least one of theswitching element Q₁ to the fourth switching element Q₄, and the primarywinding 101 of the transformer 101 and 102 of the main converter 100 isreferred to as a primary circuit of the main converter 100. Further, theconfiguration including at least one of the secondary winding 102 of thetransformer 101 and 102, the fifth switching element Q₅, the sixthswitching element Q₆, the sub converter 110, and the capacitor elementCo is referred to as a secondary circuit of the main converter 100.

A boost converter is shown as an example of the sub converter 110. Asshown, the sub converter 110 may include an inductor element Lo, thefirst sub switching element Q_(B1) and the second sub switching elementQ_(B2).

More specifically, the sub converter 110 may be disposed between bothterminals of secondary winding 102 of the transformer 101 and 102 andboth terminals of the capacitor element Co.

The current flow from the terminals of the secondary winging 102 of thetransformer to the load may be adjusted by the sub switching elementsQ_(B1) and Q_(B2).

According to an embodiment of the present invention, in a nominal mode,the second sub switching element Q_(B2) may repeatedly be turned onwhile the first sub switching element Q_(B1) may repeatedly be turnedoff. Here, the current from the secondary winding 102 of the transformerto the inductor element Lo may flow into the second sub switchingelement Q_(B2).

According to an embodiment of the present invention, in a hold-up mode,the first sub switching element Q_(B1) and the second sub switchingelement QB2 may be periodically turned on and turned off. For instance,in the hold-up mode, the first sub switching element QB1 may be turnedon when the third switching element Q₃ is turned on. Additionally, thefirst sub switching element QB1 may be turned off while the thirdswitching element QB3 remains turned on. Additionally, the second subswitching element QB2 may be turned on after a predetermined period oftime from when the first sub switching element Q_(B1) has been turnedoff. Additionally, the second sub switching element Q_(B2) may be turnedoff when the third switching element Q_(B3) is turned off. Moreover, inthe hold-up mode, the first sub switching element Q_(B1) may be turnedon when the fourth switching element Q₄ is turned on. Additionally, thefirst sub switching element Q_(B1) may be turned off while the fourthswitching element Q_(B4) remains turned on. Additionally, the second subswitching element Q_(B2) may be turned on after a predetermined periodof time from when the first sub switching element Q_(B1) has been turnedoff. Additionally, the second sub switching element Q_(B2) may be turnedoff when the fourth switching element Q_(B4) is turned off.

Hereinafter, an operational principle of a power converter integratingwith an auxiliary converter according to an embodiment of the presentinvention will be described in detail with reference to FIGS. 2 and 5.

FIG. 2 is a graph illustrating operational wave forms of main componentsof a circuit in a nominal mode.

FIG. 3 is a graph illustrating operational wave forms of main componentsof a circuit in a hold-up mode.

FIGS. 4A to 4B are circuit diagrams illustrating the operation of acircuit in a nominal mode.

FIGS. 5A to 5D are circuit diagrams illustrating the operation of acircuit in a hold-up mode.

In the FIGS. 4A to 4C and 5A to 5D, components not operating arerepresented by dotted lines.

Referring to FIGS. 2 and 4A to 4C, a nominal mode may be divided into afirst period (t0 to t1), a second period (t1 to t2), a third period t2to t3, a fourth period t3 to t4, a fifth period t4 to t5, and a sixthperiod t5 to t6. The operational principle of the fourth period t3 to t4through the sixth period t5 to t6 are same as that of the first periodt0 to t1 through the third period t2 to t3; and, therefore, the firstperiod t0 to t1 through the third period t2 to t3 will be mainlydescribed for simplicity and clarity.

1. First Period t0 to t1—Q₁/Q₃/Q₅/Q_(B2): ON, Q₂/Q₄/Q₆/Q_(B1): OFF (seeFIG. 4A)

Since the first switching element Q₁ and the third switching element Q₃are in the on state, a voltage V_(pri) in the primary winding 101 of themain converter 100 is equal to voltage source Vs. Therefore, a primarycurrent i_(pri) flowing through a path from the first switching elementQ₁, to the primary winding 101 of the transformer, and to the thirdswitching element Q₃ is increased at a constant gradient. Further, sincethe fifth switching element Q₅ is in the on state, a voltage Vrec2 inthe secondary winding 102 becomes a voltage of Vs/n according to a turnsratio (n:1), such that a current i_(Lo) flowing into the inductor Lo isincreased at a gradient of (Vs/n−Vo)/LO.

Each of parasitic capacitors C2 and C4 of the second switching elementQ₂ and the fourth switching element Q₄ is charged with the voltage ofVs. As described above, in the first period, the main power is poweredfrom the primary circuit of the main converter 100 to the secondarycircuit thereof. Other symbols i_(Q5) and i_(Q6) that are not described,respectively, denote a current flowing in the fifth switching element Q₅and a current flowing in the sixth switching element Q₆.

2. Second Period t1 to t2—Q₃/Q₅/Q_(B2): ON, Q₁/Q₄/Q_(B1): OFF, Q₂/Q₆:Turned on (See FIG. 4B)

During this period, the second switching element Q₂ and the sixthswitching element Q₆ are turned on. After the voltage charged in theparasitic capacitor C2 of the second switching element Q₂ in the firstperiod is completely discharged, the second switching element Q₂ isturned on, such that the zero voltage switching may be performed for thesecond switching element Q₂.

The voltage V_(pri) in the primary winding 101 of the main converter 100is 0V, such that the primary current i_(pri) flows through a path fromthe second switching element Q₂, to the primary winding 101 of thetransformer, and to the third switching element Q₃. Since the voltageV_(pri) in the primary wiring 101 of the main converter 100 is 0V, thevoltage Vrec in the secondary winding 102 is also 0V.

Here, the current i_(Lo) flowing into the inductor Lo of the mainconverter 100 flows through the fifth switching element Q₅ and the sixthswitching element Q₆. Here, a gradient of the current i_(Lo) is Vo/Lo.

3. Third Period t2 to t3—Q₂/Q₆/Q_(B2): ON, Q₁/Q₄/Q_(B1): OFF, Q₃/Q₅:Turned Off (See FIG. 4C)

In this period, the third switching element Q₃ and the fifth switchingelement Q₅ are turned off. Since the third switching element Q₃ isturned off, the voltage charged in the fourth switching element Q₄ iscompletely discharged through a path from the voltage source Vs, to thesecond switching element Q₂, and to the primary winding 101 of thetransformer, and the voltage V_(pri) in the primary wiring 101 of themain converter 100 decreases from 0V to −Vs. As the voltage in theprimary winding 101 of the transformer decreases from 0V to −Vs, theprimary current i_(pri), flowing in the primary winding 101 of thetransformer is also reduced accordingly. In addition, when the fifthswitching element Q₅ is turned off, the current, which was flowingthrough the channel of the fifth switching element Q₅, flows through thediode D5.

The sum of the current i_(Q5) flowing through the diode D5 of the fifthswitching element Q₅ and the current i_(Q6) flowing through the body ofthe sixth switching element Q₆ is equal to the current i_(Lo) flowing inthe inductor Lo of the main converter 100. As shown, the voltage Vrec inthe second winding 102 of the main converter 100 is 0V.

In FIG. 2, the symbol Ts denotes a period of time of one switchingcycle. ADC conversion ratio of the power supply according to the presentembodiment in the normal mode, may be expressed by equation 1.

$\begin{matrix}{\frac{V_{O}}{V_{S}} = {2\; D_{FB} \times \frac{N_{S}}{N_{P}}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here, DFB denotes an effective duty ratio of a phase shifted full bridgeDC to DC converter, while Vs denotes an input voltage, and Vo denotes anoutput voltage. Further, Np denotes a turns amount of the primarywinding of a transformer, and Ns denotes turns amount of the secondarywinding.

Referring to FIGS. 3 and 5, a hold-up mode may be mainly divided into afirst period (t0 to t1), a second period (t1 to t2), a third period t2to t3, a fourth period t3 to t4, a fifth period t4 to t5, a sixth periodt5 to t6, a seventh period t6 to t7, and an eighth period t7 to t8. Theoperational principle of the fifth period t4 to t5 through the eighthperiod t7 to t8 is the same as that of the first period t0 to t1 throughthe fourth period t3 to t4; and, therefore, the first period t0 to t1through the fourth period t3 to t4 will be mainly described.

1. First Period t0 to t1—Q₁/Q₃/Q₅/Q_(B1): ON, Q₂/Q₄/Q₆/Q_(B2): OFF (seeFIG. 5A)

Since the first switching element Q₁ and the third switching element Q₃are in the on state, a voltage V_(pri) in the primary winding 101 of themain converter 100 is equal to that of a voltage source Vs. Therefore, aprimary current i_(pri) flowing through a path from the first switchingelement Q₁, to the primary winding 101 of the transformer, and to thethird switching element Q₃ is increased at a constant gradient. Further,since the fifth switching element Q₅ is in the on state and the firstsub switching element Q_(B1) is in the on state, a voltage Vrec in thesecondary winding 102 becomes a voltage of Vs/n according to a turnsratio (n:1), such that a current i_(Lo) flowing into the inductor Lo isincreased at a gradient of (Vs)/nLO.

Each of parasitic capacitors C2 and C4 of the second switching elementQ₂ and the fourth switching element Q₄ is charged with the voltage ofVs. As described above, in the first period, the main power is poweredfrom the primary circuit of the main converter 100 to the secondarycircuit thereof. Further, in the first period, the first sub switchingelement Q_(B1) is in the on-state, and thus the energy powered to thesecondary circuit may be stored in the inductor Lo.

Other symbols i_(Q5) and i_(Q6) that are not described, respectively,denote a current flowing in the fifth switching element Q₅ and a currentflowing in the sixth switching element Q₆.

2. Second Period t1 to t2—Q₁/Q₃/Q₅/Q_(B2): ON, Q₂/Q₄/Q₆/Q_(B1): OFF (seeFIG. 5B)

Like in the first period, in the second period, the main power ispowered from the primary circuit of the main converter 100 to thesecondary circuit thereof.

In the second period, however, the first sub switching element Q_(B1)may be turned off and the second sub switching element Q_(B2) may beturned on. Therefore, the energy stored in the inductor Lo may bedelivered to the load.

3. Third Period t2 to t3—Q₃/Q₅/Q_(B2): ON, Q₁/Q₄/Q_(B1): OFF, Q₂/Q₆:Turned on (see FIG. 5C)

During this period, the second switching element Q₂ and the sixthswitching element Q₆ are turned on. After the voltage charged in theparasitic capacitor C2 of the second switching element Q₂ in the firstperiod is completely discharged, the second switching element Q₂ isturned on, such that the zero voltage switching may be performed for thesecond switching element Q₂.

The voltage V_(pri), in the primary winding 101 of the main converter100 is 0V, such that the primary current i_(pri), flows through a pathfrom the second switching element Q₂, to the primary winding 101 of thetransformer, and to the third switching element Q₃. Since the voltageV_(pri) in the primary wiring 101 of the main converter 100 is 0V, thevoltage Vrec in the secondary winding 102 is also 0V.

Accordingly, the zero voltage switching of a sub switching element maybe performed, and the current i_(Lo) flowing in the inductor Lo of themain converter 100 flows through the fifth switching element Q₅, and thesixth switching element Q₆. Here, a gradient of the current i_(Lo) isVo/Lo.

4. Fourth Period t3 to t4—Q₂/Q₆/Q_(B2): ON, Q₁/Q₄/Q_(B1): OFF, Q₃/Q₅:Turned off (see FIG. 5D)

In this period, the third switching element Q₃ and the fifth switchingelement Q₅ are turned off. Since the third switching element Q₃ isturned off, the voltage charged in the fourth switching element Q₄ iscompletely discharged through a path from the voltage source Vs, to thesecond switching element Q₂, and to the primary winding 101 of thetransformer, and the voltage V_(pri) in the primary wiring 101 of themain converter 100 decreases from 0V to −Vs. As the voltage in theprimary winding 101 of the transformer decreases from 0V to −Vs, theprimary current i_(pri) flowing in the primary winding 101 of thetransformer is also reduced accordingly. In addition, since the fifthswitching element Q₅ is turned off, the current, which was flowingthrough the channel of the fifth switching element Q₅, flows through thediode D5.

The sum of the current i_(Q5) flowing through the diode D5 of the fifthswitching element Q₅ and the current i_(Q6) flowing through the body ofthe sixth switching element Q₆ is equal to the current i_(Lo) flowing inthe inductor Lo of the main converter 100. As shown, the voltage Vrec inthe second winding 102 of the main converter 100 is 0V.

In FIG. 3, the symbol Ts denotes a period of time of one switchingcycle. ADC conversion ratio of the power supply according to the presentembodiment in the normal mode, may be expressed by equation 2.

$\begin{matrix}{\frac{V_{O}}{V_{S}} = {\left( {2\; D_{FB} \times \frac{N_{S}}{N_{P}}} \right) \times \frac{D_{B}}{1 - D_{B}}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Wherein the DFB denotes an effective duty ratio of a phase shifted fullbridge DC to DC converter. And, wherein Vs denotes an input voltage, andVo denotes an output voltage. Further, Np denotes a turns amount of theprimary winding of a transformer, and Ns denotes turns amount of thesecondary winding. In addition, DB denotes a duty ratio of the first subswitching element of the sub converter.

According to an embodiment of the present invention, the sub converter110 included in the power supply may perform a boost function. The subconverter 110 may be operable during a hold-up period in which AC lossoccurs, so that it may retain the output voltage.

Further, in a nominal mode, the sub converter 110 may retain the firstsub switching element Q_(B1) in an off state continually, whileretaining the second sub switching element in an on state continuously,thereby minimizing additional loss.

Accordingly, the PSFB, according to the embodiment of the presentinvention, allows the duty ratio to be designed to have a relativelyhigh value.

The embodiments of the present invention have been described withreference to the accompanying drawings. Although the embodiment of thepresent invention has been described in which the phase shifted DC to DCfull-bridge converter is the main converter and the boost converter isthe auxiliary converter, this is merely illustrative. It should be notedthat various types of DC to DC converters other than the phase shiftedDC to DC full-bridge converter and the boost converter may be used asthe main converter and the auxiliary converter.

As set forth above, according to embodiments of the present invention,the efficiency of a power supply can be improved by controlling outputvoltage during a hold-up time.

While the present invention has been shown and described in connectionwith the embodiments, it will be apparent to those skilled in the artthat modifications and variations can be made without departing from thespirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A power supply, comprising: a direct current (DC)to DC converter supplying main power to a load; and a sub converterconnected to the DC to DC converter and reducing output loss, whereinthe sub converter is operable based on a hold-up time.
 2. The powersupply of claim 1, wherein the DC to DC converter includes: a primarycircuit including a primary winding of a transformer; and a secondarycircuit including a second winding magnetically coupled to the primarywinding of the transformer.
 3. The power supply of claim 2, wherein theprimary circuit of the DC to DC converter includes a switching module inwhich two terminals, one terminal of a first switching element and oneterminal of a second switching element connected in series, areconnected to both terminals of a voltage source, and two terminals, oneterminal of a third switching element and one terminal of a fourthswitching element connected in series, are connected to the bothterminals of the voltage source in parallel, wherein the primary windingof the transformer is connected between a first node and a second node,wherein the first switching element and the second switching element areconnected at the first node, and the third switching element and thefourth switching element are connected at the second node.
 4. The powersupply of claim 3, wherein the secondary circuit of the DC to DCconverter includes a switching element connected to the secondarywinding of the transformer in series, and controlling current flowing inthe secondary winding of the transformer.
 5. The power supply of claim2, wherein the sub converter includes: an inductor element connected tothe secondary circuit; a first sub switching element providing a pathfor energy to be stored in the inductor element; and a second subswitching element providing a path for delivering energy stored in theinductor element.
 6. The power supply of claim 5, wherein the first subswitching element is turned on when the third switching element isturned on, and is turned off while the third switching element remainsturned on.
 7. The power supply of claim 6, wherein the second subswitching element is turned on after a predetermined period of time fromwhen the first sub switching element is turned off, and is turned offwhen the third switching element is turned off.
 8. The power supply ofclaim 5, wherein the first sub switching element is turned on when thefourth switching element is turned on, and is turned off while thefourth switching element remains turned on.
 9. The power supply of claim8, wherein the second sub switching element is turned on after apredetermined period of time from when the first sub switching elementis turned off, and is turned off when the fourth switching element isturned off.
 10. A power supply, comprising: a DC to DC convertersupplying main power through a primary circuit connected to terminals ofa voltage source and a secondary circuit magnetically coupled to theprimary circuit; and a sub converter storing energy supplied from thesecondary circuit and providing a path for delivering the stored energy.